The primary components of most vegetable oils are triglycerides. Depending on the oil source, these triglycerides may include a variety of saturated, partially saturated, and unsaturated fatty acids (e.g., oleic, linoleic, linolenic, lauric, palmitic, and myristic acids) esterified on a glycerol molecule in various combinations. Raw, unprocessed vegetable oils often contain varying amounts of other compounds, too. Some of these compounds are desirable components of the oil, i.e., they need not be removed during processing to yield a commercially salable oil. Such desirable components commonly include diglycerides, tocopherols, sterols, and sterol esters, and some oils will include other desirable components (e.g., tocotrienols in palm oil). Other compounds in the raw vegetable oil are undesirable impurities which can adversely affect the taste, smell, appearance, or storage stability of a refined oil and hence are beneficially removed. These undesirable impurities generally include phosphatides, free fatty acids, odiferous volatiles, colorants, waxes, and various metal compounds. Some of these undesirable impurities (e.g., metal compounds) are contaminants which have little or no commercial value. Free fatty acids and some other components of raw oils are undesirable in a processed oil and may be considered “undesirable impurities” in that context, but may still have meaningful commercial value.
Vegetable oil impurities are typically removed in three separate steps, particularly degumming and/or alkali refining, bleaching, and physical refining and/or deodorizing. For certain oils, it may be desirable to “degum” the raw oil prior to alkali refining. Degumming and alkali refining generally remove phosphatides, soaps, and other impurities such as metals. Bleaching may improve the color and flavor of refined oil by decomposing peroxides and removing oxidation products, trace phosphatides, and trace soaps. In the final physical refining and/or deodorizing step, remaining volatilizable impurities are removed to yield a deodorized vegetable oil having the desired final characteristics. The volatilizable impurities removed in the deodorization process commonly include free fatty acids, aldehydes, ketones, alcohols, and other hydrocarbon impurities. Some of these impurities may come directly from the oil seeds themselves while others may arise from pesticides, fungicides, and other compounds applied to the seeds or to the plants from which the seeds are derived.
Most physical refining and deodorization processes rely on volatility differences to drive off the relatively more volatile impurities from the relatively less volatile desirable components of the vegetable oil. Unfortunately, some of the desirable components of the vegetable oil may be driven off with the volatilizable impurities during the deodorization process. As a consequence, the volatiles stream exiting the deodorizer will include a substantial fraction, if not substantially all, of the impurities, but may also include a varying amount of desirable compounds such as triglycerides, diglycerides, tocopherols, sterols, sterol esters, and tocotrienols. Since these desirable compounds could remain in the final deodorized vegetable oil without objection, the volume of such desirable compounds in the vapor stream exiting the deodorizer represents loss of a commercially valuable product.
Most conventional deodorizing processes are carried out using vacuum-steam deodorization, in which a heated vegetable oil is contacted with steam at a low operating pressure. The volatilizable impurities and some of the desirable vegetable oil components are carried off with the steam. This steam-laden vapor stream is subsequently condensed into an impurity-laden liquid, with some or all of the steam being vented to the atmosphere through the vacuum system. Other deodorizing processes may employ heated nitrogen or other gases instead of steam, but such processes are less widely used in vegetable oil processing.
Because of the relatively high concentration of impurities, the condensate from the deodorizer vapor stream has limited commercial value. While it is theoretically possible to reintroduce this condensate into the raw oil feed, this is often ill-advised because it can unacceptably degrade the quality of the starting oil and overburden the capacity of the deodorizer. It may be possible to resell the condensate for further processing to use as an animal feed additive or the like, but the value of such a condensate is significantly less than the value of the deodorized vegetable oil.